UNPRECEDENTED levels of testosterone were pumping through the body of the young male elephant which crushed and almost killed one of Taronga Zoo's keepers last year, an investigation by the zoo has revealed.

Two-year-old Pathi Harn had higher levels of the male hormone than Taronga's five year-old adult male Gung, acting general manager of research and conservation, Dr Rebecca Spindler, said this morning.

Two of Taronga's female elephants were on heat at the time 1000 kilogram Pathi Harn pinned 40-year-old keeper Lucy Melo against a bollard in the elepehant barn in October last year.

"It's entirely possible this spike in testosterone contributed to the sudden changes in behaviour we saw that day," Dr Spindler said.

Ms Melo's heart stopped for five minutes after she was crushed and she was revived by paramedics and spent 12 days in hospital with fractured ribs.

Ms Meo returned to work at the zoo today at the elephant enclosure, but will not do any public appareances.

Dr Spindler said said neither the zoo or the trainer or the elephant calf Pathi Harn had been at fault.

Dr Spindker said keeprs had been handling elephants with the proper procedures, and the design of the elephant barn had not contributed to the incindent.

But she said the zoo's rules for dealing with adult male elephants would now be used on Pathi Harn and its other young male, three and a half year old Luk Chai.

This incolved keepers always making sure there was a bollard or pole between them and the young elephants.

Dr Spindler said Pathi Harn was apparently the fastest sexually developing male elephant in the world, but the zoo did not know why and also did not know if it was connected with the elephant's "miracle birth".

He was born two days after being pronounced dead in the womb and nicknamed Mr Shuffles for the way he walked - and was reduced to the bottom of the zoo elephant herd hierachy.

But Dr Spindler said although Pathi had been boisterous leading up to the incident, he had not been aggressive when Ms Melo was crushed.

"He just seemed to move his head about quickly and catch her with his trunk, but stopped straight away when the other keepers responded, and went back to eating hay.'' she said.